The present invention relates to a method of aluminum diffusion layer formation to be applied to various components or parts in which resistances to oxidization and heat are required, which method is particularly suitable for application to components or parts, for example, metallic products such as plate members, containers, pipes, etc., to be satisfactorily usable in the presence of elevated temperatures and strongly corrosive medium generally containing halogen gas or halogen compounds, and especially for parts in the exhaust systems of motor vehicles such as reactors, silencers and the like.
Conventionally, in the formation of aluminum or Al diffusion layers, there has been proposed a solid diffusion method (so called pack method) known as calorizing, for example, by U.S. Pat. Nos. 3,079,276 and 3,096,160 entitled "Vapor diffusion coating process", in which workpieces to be treated are buried in aluminum particles or powder of various metals or salts and are heated at high temperatures for a long time under reducing atmosphere to generate vapor of aluminum to be penetrated into the workpieces for consequent formation of Al diffusion layers on the surfaces of such workpieces.
The conventional Al diffusion layer forming method as described above, however, has such a disadvantage that not only facilities are required for maintaining the reducing atmosphere, but a considerable period of time, for example, two to four hours is required for the treatment, thus resulting in high cost of the final products.
There has also been conventionally proposed a melt plating method called "aluminizing", for example, by U.S. Pat. No. 3,907,611 entitled "Method of making ferrous metal having highly improved resistances to corrosion at elevated temperatures and to oxidization" and assigned to the assignee of the present invention. The known method as described above, however, is not intended to form the diffusion layer of Al, but to form Al plating layers or Al compound layers on the surfaces of workpieces to be treated, and is incapable of forming only the Al duffusion layer on the surface of the ferrous alloy workpiece as proposed in the present invention.